This foot doesn't pound

"It's like the difference between walking with a ski boot on and walking in tennis shoes."

Mount Sterling, OH— The engineers at Ohio Willow Wood know that nature sometimes trumps technology. Advanced materials and bio-mechanics expertise have yet to help them create artificial limbs that fully restore what disease or accident takes away. But they recently took a giant step toward that goal with a patented prosthetic foot that mimics the smooth motion of a natural foot and ankle. "Amputees tell us it's the closest we've ever come to replacing what they've lost," says Jim Colvin, the company's engineering director.

A far cry from the simple wooden legs evoked by the company's name, the new Pathfinder foot combines glass-and-carbon-reinforced epoxy components with a pneumatic shock absorber. Two strip-like composite springs run from "toe" to "shank," defining the top curve of the foot. At the shank's lower end, an air-and-nitrogen shock sits where the heel would, soaking up vertical loads that normally range from1.5 to 5 times body weight. And a flexible composite plate creates a "sole" that connects the springs to the heel shocks to complete that Pathfinder's closed-triangle configuration.

Pathfinder's biggest stride toward a more natural step comes from this connection between the foot's compressive and flexible components. Other top-of-the-line feet employ composite toe springs and heel shocks too. "But these designs don't tie them together with a sole plate," Colvin says, explaining that the Pathfinder's interconnected components have to work in unison to replicate the complex dynamic characteristics and motion of the human foot and ankle.

A step forward. Like the anatomy it replaces, the Pathfinder actively propels users through a step as the individual components capture, store, and release energy in an overlapping sequence. "The foot doesn't just drag along like dead weight," Colvin notes. The sequence begins at heel strike as the shock absorbs impact forces. "Rebound from the compressed heel shocks propels users forward," says Colvin. As the gait progresses, the toe springs progressively take over the energy-return chores—capturing energy as they deflect and gradually releasing it as they straighten to complete the step. The sole plate, too, flexes to aid the transition from heel to toe.

The Pathfinder prosthetic foot employs interconnected flexible and comopressive elemetns to smooth the transition from heal to toe loading.

According to Jeff Doddroe, the engineer who designed Pathfinder, these energy-return characteristics produce a motion that closely approximates that of a natural foot and ankle. He describes the motion as "polycentric"—meaning that Pathfinder's center of rotation shifts as the gait cycle advances. And it's this motion that creates a smooth transition from heel-to-toe loading as opposed to two distinct impacts, or "bumps," exhibited by feet with decoupled heel shocks and springs.

The heel-to-toe connection also helps create a more stable foot. As Colvin explains, loads that compress the heel shocks also apply a moment to the toe springs, pushing them open and driving the toe down soon after heel strike. "The entire sole of the foot comes into contact with the ground much earlier in the gait cycle, which enhances stability," he says. Unconnected heel shocks and toe springs, by contrast, would offer no such approximation of a natural foot's flexion until much later in the gait cycle. "Before, the spring wouldn't come into play until the foot goes flat at mid-stance," Colvin says. A heel-to-toe split in the sole plate adds a bit more stability by allowing the sole to conform to uneven surfaces.

The sole plate also works to enhance the foot's durability by physically limiting the deflection of the springs. With a decoupled heel and toe, the composite springs extend as cantilevers from the foot's shank. "Oftentimes they would bend too much and delaminate," Colvin adds.

Foot pounds. Given its dynamic nature and the pounding that artificial feet take, the Pathfinder's materials tread that very fine line between strength and flexibility. "Composites with the right response for an active person didn't always hold up long enough," Colvin says.

The Pathfinder has to support users who weigh from 150 to 250 lbs—and remember, vertical loads often reach five times body weight. It also has to withstand a fatigue test that subjects it to an alternate loading of heel and toe to 340 lbs for two million cycles. And it goes through an impact test that drops a loaded foot mounted at a 20-degree angle on a rigid plate from progressively higher heights until the springs fail. The Pathfinder made it to 16 inches with a 165- lb load compared to just 11 inches for an earlier foot with composite springs—a difference that translates to 50% more impact energy before failure. This impact performance corresponds directly to the amount of impact forces transmitted into the body, so that the Pathfinder transmits less than half as much force as the earlier leg does.

The Pathfinder's composites also had to have enough flexibility to deliver the energy-return characteristics that users equate with performance. Ohio Willow Wood's energy-return tests—which involve displacement measurements during a progressive loading and unloading cycle—show that Pathfinder consistently achieves an 87 to 91% efficiency. An earlier foot with a composite toe spring and separate shock returned a lower and more variable efficiency of just 47 to 84%.

To meet these tough strength and flexibility requirements, Ohio Willow Wood engineers tapped their materials knowledge. Colvin won't reveal too much about the composites Ohio Willow Wood uses, other than to say the sole plate and toe springs are produced from the same resin system. Only the orientation, position, and proportion of the glass and carbon-fiber reinforcements vary. "The real trick is in the lay-up," he says, noting that the materials development process took more than two years. The company produces the composite components in a range of stiffnesses—just one of the strategies that allow prosthetists to tailor the foot to individual walking styles and activity levels.

Finding the right shocks proved difficult too. After looking at solutions ranging from an elastomer shock to composite springs, Doddroe ultimately found shocks that met size, weight, and response requirements in an unlikely place—on a mountain bike. In another critical part of the how the Pathfinder can be adapted to individual user needs, the foot comes with a hand pump that lets users adjust the shock pressure from 30 to 200 psi.

At 858 grams for a 26-cm foot, the Pathfinder does weight about 300 to 500 grams more than other top-of-the-line feet. But in the prosthetics business, perception sometimes counts for more than actual weight. "Many users tell us it feels lighter," says Colvin, attributing that perception to the fatigue reduction that results from improved energy return and lower impact forces.

Walking the walk. As much as mechanics and materials matter to engineers, amputees care far more about a foot that feels as natural as possible. Lonnie Nolt cares about both things. A research engineer for Ohio Willow Wood and an amputee for the last year-and-a-half, he recently began using a Pathfinder after trying a succession of less advanced foot designs. And he immediately noticed that Pathfinder eliminates the "dead spot" or bumpy transition between heel and toe loading. "That makes a huge difference in comfort," he says, "It's like the difference between walking with a ski boot on and walking in tennis shoes."

Jeff Doddroe

Jeff Doddroe is a staff engineer in Ohio Willow Wood's R&D department. He has been at the company for 20 years and has been involved with every type of prosthetic foot the company makes. Before moving into research, he worked in the company's plastics manufacturing operation.

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